UDP报文GSO分段的入口函数是udp4_ufo_fragment,由udpv4_offload常量中定义。
1、udp4_ufo_fragment函数
static struct sk_buff *udp4_ufo_fragment(struct sk_buff *skb, netdev_features_t features) { struct sk_buff *segs = ERR_PTR(-EINVAL); unsigned int mss; __wsum csum; struct udphdr *uh; struct iphdr *iph; if (skb->encapsulation && (skb_shinfo(skb)->gso_type & (SKB_GSO_UDP_TUNNEL|SKB_GSO_UDP_TUNNEL_CSUM))) { segs = skb_udp_tunnel_segment(skb, features, false); //封装报文的GSO分段,可以基于内层报文进行GSO分段 goto out; } if (!pskb_may_pull(skb, sizeof(struct udphdr))) goto out; mss = skb_shinfo(skb)->gso_size; if (unlikely(skb->len <= mss)) goto out; if (skb_gso_ok(skb, features | NETIF_F_GSO_ROBUST)) { /* Packet is from an untrusted source, reset gso_segs. */ int type = skb_shinfo(skb)->gso_type; if (unlikely(type & ~(SKB_GSO_UDP | SKB_GSO_DODGY | SKB_GSO_UDP_TUNNEL | SKB_GSO_UDP_TUNNEL_CSUM | SKB_GSO_TUNNEL_REMCSUM | SKB_GSO_IPIP | SKB_GSO_GRE | SKB_GSO_GRE_CSUM) || !(type & (SKB_GSO_UDP)))) goto out; skb_shinfo(skb)->gso_segs = DIV_ROUND_UP(skb->len, mss); //如果报文来源不可信,则重新计算segs,返回 segs = NULL; goto out; } /* Do software UFO. Complete and fill in the UDP checksum as * HW cannot do checksum of UDP packets sent as multiple * IP fragments. */ uh = udp_hdr(skb); iph = ip_hdr(skb); uh->check = 0; csum = skb_checksum(skb, 0, skb->len, 0); //计算csum值 uh->check = udp_v4_check(skb->len, iph->saddr, iph->daddr, csum); //计算udp头的check值 if (uh->check == 0) uh->check = CSUM_MANGLED_0; skb->ip_summed = CHECKSUM_NONE; /* Fragment the skb. IP headers of the fragments are updated in * inet_gso_segment() */ segs = skb_segment(skb, features); //报文根据mss进行分段,因为包含UDP头,所以分段的结果是IP分片报文 out: return segs; }2、skb_udp_tunnel_segment函数
struct sk_buff *skb_udp_tunnel_segment(struct sk_buff *skb, netdev_features_t features, bool is_ipv6) { __be16 protocol = skb->protocol; const struct net_offload **offloads; const struct net_offload *ops; struct sk_buff *segs = ERR_PTR(-EINVAL); struct sk_buff *(*gso_inner_segment)(struct sk_buff *skb, netdev_features_t features); rcu_read_lock(); switch (skb->inner_protocol_type) { //vxlan封装时,该值为ENCAP_TYPE_ETHER case ENCAP_TYPE_ETHER: protocol = skb->inner_protocol; gso_inner_segment = skb_mac_gso_segment; //vxlan封装,内层报文为完整的报文(二层、三层、四层),继续从mac开始分段 break; case ENCAP_TYPE_IPPROTO: offloads = is_ipv6 ? inet6_offloads : inet_offloads; ops = rcu_dereference(offloads[skb->inner_ipproto]); if (!ops || !ops->callbacks.gso_segment) goto out_unlock; gso_inner_segment = ops->callbacks.gso_segment; //调用4层协议的GSO分段能力,GRE/IPIP等等 break; default: goto out_unlock; } segs = __skb_udp_tunnel_segment(skb, features, gso_inner_segment, //upd封装报文GSO分段 protocol, is_ipv6); out_unlock: rcu_read_unlock(); return segs; }3、__skb_udp_tunnel_segment函数
static struct sk_buff *__skb_udp_tunnel_segment(struct sk_buff *skb, netdev_features_t features, struct sk_buff *(*gso_inner_segment)(struct sk_buff *skb, netdev_features_t features), __be16 new_protocol, bool is_ipv6) { struct sk_buff *segs = ERR_PTR(-EINVAL); u16 mac_offset = skb->mac_header; int mac_len = skb->mac_len; int tnl_hlen = skb_inner_mac_header(skb) - skb_transport_header(skb); //vxlan头长度 UDP + vxlan, __be16 protocol = skb->protocol; netdev_features_t enc_features; int udp_offset, outer_hlen; unsigned int oldlen; bool need_csum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_UDP_TUNNEL_CSUM); bool remcsum = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TUNNEL_REMCSUM); bool offload_csum = false, dont_encap = (need_csum || remcsum); oldlen = (u16)~skb->len; if (unlikely(!pskb_may_pull(skb, tnl_hlen))) goto out; skb->encapsulation = 0; __skb_pull(skb, tnl_hlen); //报文移动到内层报文的MAC头 skb_reset_mac_header(skb); //设置skb的mac header skb_set_network_header(skb, skb_inner_network_offset(skb)); //设置skb的 ip header skb->mac_len = skb_inner_network_offset(skb); //设置skb mac len skb->protocol = new_protocol; //设置skb protocol,至此skb已经切换到内层,可以继续进行GSO分段 skb->encap_hdr_csum = need_csum; skb->remcsum_offload = remcsum; /* Try to offload checksum if possible */ offload_csum = !!(need_csum && (skb->dev->features & (is_ipv6 ? NETIF_F_V6_CSUM : NETIF_F_V4_CSUM))); /* segment inner packet. */ enc_features = skb->dev->hw_enc_features & features; segs = gso_inner_segment(skb, enc_features); //如果是vxlan报文,则重新开始mac层的GSO分段 if (IS_ERR_OR_NULL(segs)) { skb_gso_error_unwind(skb, protocol, tnl_hlen, mac_offset, mac_len); goto out; } outer_hlen = skb_tnl_header_len(skb); //计算外层报文的长度 udp_offset = outer_hlen - tnl_hlen; //外层UDP头的偏移 skb = segs; //此时skb指向内层报文的mac头位置 do { struct udphdr *uh; int len; __be32 delta; if (dont_encap) { skb->encapsulation = 0; skb->ip_summed = CHECKSUM_NONE; } else { /* Only set up inner headers if we might be offloading * inner checksum. */ skb_reset_inner_headers(skb); //此时skb指向内层报文,可以建立inner header值 skb->encapsulation = 1; } skb->mac_len = mac_len; skb->protocol = protocol; skb_push(skb, outer_hlen); //skb移到外层报文的mac头 skb_reset_mac_header(skb); //设置mac header skb_set_network_header(skb, mac_len); //设置network header,ip层需要 skb_set_transport_header(skb, udp_offset); //设置transport header len = skb->len - udp_offset; uh = udp_hdr(skb); //找到UDP头很重要,GSO分段后,有些数据需要刷新,包括长度等 uh->len = htons(len); if (!need_csum) continue; delta = htonl(oldlen + len); uh->check = ~csum_fold((__force __wsum) ((__force u32)uh->check + (__force u32)delta)); if (offload_csum) { skb->ip_summed = CHECKSUM_PARTIAL; skb->csum_start = skb_transport_header(skb) - skb->head; //重新计算csum值,位置更新了 skb->csum_offset = offsetof(struct udphdr, check); } else if (remcsum) { /* Need to calculate checksum from scratch, * inner checksums are never when doing * remote_checksum_offload. */ skb->csum = skb_checksum(skb, udp_offset, //软件计算csum值 skb->len - udp_offset, 0); uh->check = csum_fold(skb->csum); if (uh->check == 0) uh->check = CSUM_MANGLED_0; } else { uh->check = gso_make_checksum(skb, ~uh->check); //计算伪头check值 if (uh->check == 0) uh->check = CSUM_MANGLED_0; } } while ((skb = skb->next)); out: return segs; }udp4_ufo_fragment提升了对vxlan等封装报文的支持,对GSO报文封装后报文的能够正确GSO分段,而不会产生IP分片报文。 实现这个功能,离不开skb_segment函数的支持,能够复制外层报文头而不管有多长,所以只要少量修改,内核能够支持多层封装的GSO分段。